Spurgeon lists the reasons that hinder our efforts in preaching the gospel to win souls

Indifference
"I would far rather have a man an earnest, intense opposer of the gospel than have him careless and indifferent. "
Unbelief
"These people are very hard to influence; but I would warn you not to fight them with their own weapons. I do not believe that infidels ever are won by argument; or, if so, it very seldom happens. The argument that convinces men of the reality of religion, is that which they gather from the holiness and earnestness of those who profess to be Christ's followers. "
False security
"We ought so to preach as to make every sinner tremble in his seat; and if he will not come to the Saviour, he ought at least to have a hard time of it while he stops away from Him. I am afraid that we sometimes preach smooth things, too soothing and agreeable, and that we do not set before men their real danger as we should. If we shun in this respect to declare all the counsel of God, part at least of the responsibility of their ruin will lie at our door."
Despair
"It is fatalism, not predestination, that makes men talk as if there is nothing whatever for them to do, or that there is nothing they can do. There is no likelihood of anyone being saved while he gives you this as his only hope, "If salvation is for me, it will come to me in due time."
The love of sin.
Self righteousness
"That is the real difficulty in many, many cases; the man does not come to Christ because he is not conscious that he is lost, he does not ask to be lifted up because he does not know that he is a fallen creature, he does not feel that he has any need of divine mercy or forgiveness, and therefore he does not seek it."
Utter worldliness
Bad habits and bad company